1 准备测试用表

上传官方的数据集
将 visits_v1.tar 和 hits_v1.tar 上传到虚拟机，解压到 clickhouse 数据路径下

// 解压到 clickhouse 数据路径
sudo tar -xvf hits_v1.tar -C /var/lib/clickhouse
sudo tar -xvf visits_v1.tar -C /var/lib/clickhouse

//修改所属用户
sudo chown -R clickhouse:clickhouse /var/lib/clickhouse/data/datasets
sudo chown -R clickhouse:clickhouse /var/lib/clickhouse/metadata/datasets

重启 clickhouse-server
```
sudo clickhouse restart
```
执行查询
```
clickhouse-client --query "SELECT COUNT(*) FROM datasets.hits_v1"
clickhouse-client --query "SELECT COUNT(*) FROM datasets.visits_v1"
```
注意：官方的 tar 包，包含了建库、建表语句、数据内容，这种方式不需要手动建库、建表，最方便。
hits_v1 表有 130 多个字段，880 多万条数据
visits_v1 表有 180 多个字段，160 多万条数据

2 COUNT 优化

在调用 count 函数时，如果使用的是 count() 或者 count(*)，且没有 where 条件，则会直接使用 system.tables 的 total_rows，例如:

EXPLAIN SELECT count()FROM datasets.hits_v1;

Union
  Expression (Projection)
    Expression (Before ORDER BY and SELECT)
      MergingAggregated
        ReadNothing (Optimized trivial count)

注意 Optimized trivial count ，这是对 count 的优化, 只要括号中不写具体的字段就会进行优化。
如果 count 具体的列字段，则不会使用此项优化：

EXPLAIN SELECT count(CounterID) FROM datasets.hits_v1;

Union
  Expression (Projection)
    Expression (Before ORDER BY and SELECT)
      Aggregating
        Expression (Before GROUP BY)
          ReadFromStorage (Read from MergeTree)

3 消除子查询重复字段

下面语句子查询中有两个重复的 id 字段，会被去重:

EXPLAIN SYNTAX SELECT 
		a.UserID,
		b.VisitID,
		a.URL,
		b.UserID
	FROM
		hits_v1 AS a 
	LEFT JOIN ( 
		SELECT 
			UserID, 
			UserID as HaHa, 
			VisitID 
		FROM visits_v1) AS b 
		USING (UserID)
	limit 3;

返回优化语句：

SELECT 
	UserID,
	VisitID,
	URL,
	b.UserID
FROM hits_v1 AS a
ALL LEFT JOIN 
(
	SELECT 
	UserID,
	VisitID
	FROM visits_v1
) AS b USING (UserID)
LIMIT 3

4 谓词下推

??当 group by 有 having 子句，但是没有 with cube、with rollup 或者 with totals 修饰的时候，having 过滤会下推到 where 提前过滤。例如下面的查询，HAVING name 变成了 WHERE name，在 group by 之前过滤：

EXPLAIN SYNTAX 
	SELECT UserID 
	FROM hits_v1 
	GROUP BY UserID 
	HAVING UserID = '8585742290196126178';

//返回优化语句
SELECT UserID
FROM hits_v1
WHERE UserID = \'8585742290196126178\'
GROUP BY UserID

子查询也支持谓词下推：

EXPLAIN SYNTAX
SELECT *
FROM 
(
	SELECT UserID
	FROM visits_v1
)
WHERE UserID = '8585742290196126178'

//返回优化后的语句
SELECT UserID
FROM 
(
	SELECT UserID
	FROM visits_v1
	WHERE UserID = \'8585742290196126178\'
)
WHERE UserID = \'8585742290196126178\'

再来一个复杂例子：

EXPLAIN SYNTAX
	SELECT * FROM (
		SELECT * 
		FROM 
		(
			SELECT  UserID 
			FROM visits_v1
		) 
		
		UNION ALL 
		
		SELECT *
		FROM
		(
			SELECT UserID 
			FROM visits_v1
		)
	)
	WHERE UserID = '8585742290196126178'
	
//返回优化后的语句
SELECT UserID
FROM 
	(
		SELECT UserID
		FROM 
		(
			SELECT UserID
			FROM visits_v1
			WHERE UserID = \'8585742290196126178\'
		)
		WHERE UserID = \'8585742290196126178\'
		
		UNION ALL
		
		SELECT UserID
		FROM 
		(
			SELECT UserID
			FROM visits_v1
			WHERE UserID = \'8585742290196126178\'
		)
		WHERE UserID = \'8585742290196126178\'
	)
WHERE UserID = \'8585742290196126178\'

5 聚合计算外推

聚合函数内的计算，会外推，例如：

EXPLAIN SYNTAX
SELECT sum(UserID * 2)
FROM visits_v1

//返回优化后的语句
SELECT sum(UserID) * 2
FROM visits_v1

6 聚合函数消除

如果对聚合键，也就是 group by key 使用 min、max、any 聚合函数，则将函数消除，
例如：

EXPLAIN SYNTAX
SELECT
	sum(UserID * 2),
	max(VisitID),
	max(UserID)
FROM visits_v1
GROUP BY UserID

//返回优化后的语句
SELECT
	sum(UserID) * 2,
	max(VisitID),
	UserID
FROM visits_v1
GROUP BY UserID

7 删除重复的 order by key

例如下面的语句，重复的聚合键 id 字段会被去重:

EXPLAIN SYNTAX
	SELECT *
	FROM visits_v1
	ORDER BY
		UserID ASC,
		UserID ASC,
		VisitID ASC,
		VisitID ASC
	
//返回优化后的语句：
select ……
FROM visits_v1
ORDER BY 
	UserID ASC,
	VisitID ASC

8 删除重复的 limit by key

例如下面的语句，重复声明的 name 字段会被去重：

EXPLAIN SYNTAX
	SELECT *
	FROM visits_v1
	LIMIT 3 BY
		VisitID,
		VisitID
	LIMIT 10
	
//返回优化后的语句：
select ……
FROM visits_v1
LIMIT 3 BY VisitID
LIMIT 10

9 删除重复的 USING Key

例如下面的语句，重复的关联键 id 字段会被去重：

EXPLAIN SYNTAX
	SELECT
		a.UserID,
		a.UserID,
		b.VisitID,
		a.URL,
		b.UserID
	FROM hits_v1 AS a
	LEFT JOIN visits_v1 AS b USING (UserID, UserID)

//返回优化后的语句：
SELECT 
	UserID,
	UserID,
	VisitID,
	URL,
	b.UserID
FROM hits_v1 AS a
ALL LEFT JOIN visits_v1 AS b USING (UserID)

10 标量替换

如果子查询只返回一行数据，在被引用的时候用标量替换，例如下面语句中的 total_disk_usage 字段：

EXPLAIN SYNTAX
WITH 
	(
		SELECT sum(bytes)
		FROM system.parts
		WHERE active
	) AS total_disk_usage
SELECT
	(sum(bytes) / total_disk_usage) * 100 AS table_disk_usage, table
FROM system.parts
GROUP BY table
ORDER BY table_disk_usage DESC
LIMIT 10;

//返回优化后的语句：
WITH CAST(0, \'UInt64\') AS total_disk_usage
SELECT 
	(sum(bytes) / total_disk_usage) * 100 AS table_disk_usage, table
FROM system.parts
GROUP BY table
ORDER BY table_disk_usage DESC
LIMIT 10

11 三元运算优化

如果开启了 optimize_if_chain_to_multiif 参数，三元运算符会被替换成 multiIf 函数，
例如：

EXPLAIN SYNTAX 
	SELECT number = 1 ? 'hello' : (number = 2 ? 'world' : 'atguigu') 
	FROM numbers(10) 
	settings optimize_if_chain_to_multiif = 1;

//返回优化后的语句：
SELECT multiIf(number = 1, \'hello\', number = 2, \'world\', \'atguigu\')
FROM numbers(10)
SETTINGS optimize_if_chain_to_multiif = 1